Currently, engineering vehicles are normally provided with multimode steering function to cope with complicated site conditions. Here, “multimode steering” means multiple kinds of steering modes of vehicles, which usually comprises front axle independent steering, rear axle independent steering, sharp turn (also referred as “coordination”) steering, and crab type steering. During sharp turn steering, the steering directions of front axle wheels and rear axle wheels are opposite so as to achieve minimum steering radius, which is called “sharp turn steering”. During crab type steering, the steering directions of front axle wheels and rear axle wheels are same to achieve movement like a crab, which is called “crab type steering”. FIG. 1 is a schematic view showing common four steering modes.
Such multimode steering is normally carried out by manual control manipulation and electrically control manipulation. In the manual control manipulation, a manual-controlled multimode steering valve is employed, and the steering principle of its use is shown in FIG. 2. When the driver manipulates the steering wheel a4, the hydraulic oil provided by a steering pump is outputted by a full hydraulic steering device a3 from A port or B port according to the leftward or rightward turn of the steering wheel, then the hydraulic oil enters a front axle steering oil cylinder a1 and a rear axle steering oil cylinder a5 via a tandem multimode steering valve a2, so as to make wheels turn leftward or rightward.
When the valve core of the multimode steering valve a2 is located in the first position from left, the hydraulic oil outputted from the steering device only passes through the front axle steering oil cylinder, to perform front axle independent steering function. When the valve core of the multimode steering valve a2 is located in the second position from left, the hydraulic oil outputted from the steering device passes through the front axle steering oil cylinder and the rear axle steering oil cylinder in order, to make wheels turn, at this time the steering directions of front axle wheels and rear axle wheels are opposite so as to perform sharp turn steering function. When the valve core of the multimode steering valve a2 is located in the third position from left, the hydraulic oil outputted from the steering device only passes through the rear axle steering oil cylinder, to perform rear axle independent steering function. When the valve core of the multimode steering valve a2 is located in the fourth position from left, the hydraulic oil outputted from the steering device passes through the front axle steering oil cylinder and the rear axle steering oil cylinder in order, to make wheels turn, at this time the steering directions of front axle wheels and rear axle wheels are same so as to perform crab type steering function.
At present, the electrically control manipulation comprises electro-hydraulic proportional control and electromagnetic valve control. FIG. 3 shows the principle of an electro-hydraulic proportional controlled multimode steering system. The front axle wheels are controlled by the driver via the manipulation of the steering wheel, while as to the turning angle of each rear axle wheel, various electric signals are outputted by programs set in a controller according to the turning angle of a certain front axle wheel and the steering mode selected by the driver, so as to control the opening degree of electro-hydraulic proportional controlled valve and hydraulic oil flow, make steering oil cylinders push wheels steer according to the driver's intention, thus the multimode steering function is achieved. The turning angles of wheels are detected by encoders mounted on the axles.
FIG. 4 shows the principle of electromagnetic valve controlled parallel multimode steering, in which multimode selection are made by means of switching valve position of an electromagnetic valve, and the front and rear axle steering oil cylinders perform parallel actions by means of a flow distributing and collecting valve.
There are certain defects in all the three above conventional multimode steering solutions. When the manual-controlled multimode steering valve shown in FIG. 2 is switched into the sharp turn steering position or crab type steering position (i.e. when the valve core is located at the second or fourth position from the left), the front and rear axle steering oil cylinder are in tandem. After the pressure provided by the hydraulic system makes the front axle steer, the remaining pressure is required to make the rear axle steer. Thus the pressure of the front axle steering oil cylinder is so large that the sealing requirement is high, while the switching valve in manual control manipulation is complicated in structure and the valve core is special in performance, resulting in high cost.
In the electro-hydraulic proportional control multimode steering solution shown in FIG. 3, the accuracy, sensitivity and reliability of the wheel turning angle excessively depend on the precision and reliability of electric and hydraulic elements. Once the electric elements (specifically encoder, controller) fail, the steering system will not work as the driver requires, which is dangerous to high speed running vehicles. Moreover, such control system is relatively complicated in structure, needs professional staff to operate, is inconvenience in maintenance, and is relatively expensive.
The double flow distributing parallel multimode steering valves shown in FIG. 4 causes large system pressure loss due to employing two flow distributing and collecting valves. Meanwhile, during the front axle independent steering or rear axle independent steering, oil outputted by the steering device only partially enters the front (rear) axle steering oil cylinder, half or more of oil is wasted. When a sharp turn steering is carried out in these two modes, since the oil outputted by the steering device is not enough to make the flow distributing and collecting valves achieve normal flow distribution, sharp turn steering action tends to fail. However, during high speed running, sharp turn is often required to correct the direction, thus the above failure will threaten the road safety seriously.